1. Field of the Invention
The present invention relates to methods for coupling ultrasound transducers to human skin.
2. Description of the Background Art
Ultrasound is widely used both as a diagnostic tool and a therapeutic energy source. Effective coupling of an ultrasound transducer to a patient is necessary to ensure the ultrasound energy emitted by the transducer, can traverse the boundary between the transducer and the patient, without getting reflected or scattered by the boundary between the two. The presence of air between a transducer and a patient is a major problem.
Attenuation occurs when energy is lost from the ultrasound waves as they pass through a medium. The lost ultrasound energy is absorbed by the medium and generally converted into heat. Heat buildup can quickly reach dangerous levels and cause burns on a skin surface if the heat build up occurs in the coupling medium. In addition, the amount of energy transmitted into the targeted tissue is similarly reduced.
Reflection occurs when energy is deflected by a barrier. The energy that is neither absorbed nor transmitted through the barrier is deflected back toward the energy source. In ultrasound the reflection of ultrasound energy can create a secondary focal region close to the transducer face. This may result in direct or indirect damage to the transducer. Both attenuation and reflection are negative effects that are desirably avoided.
To reduce or prevent the attenuation or reflection of ultrasound energy emitted by a transducer, a “coupling medium” is usually employed between the transducer and the skin. Existing coupling agents include a variety of aqueous solutions and mineral oils. Low power ultrasound instruments, such as diagnostic imaging transducer, may safely use more viscous solutions like hydro-gels to couple the ultrasound transducer to a patient without having to compensate for issues of attenuation or reflection. As the power of a transducer increases, there is less flexibility in acceptable coupling agents. In some HIFU procedures using lower intensity treatment regimens, mineral oil and very light hydro-gels are sufficient. However when the intensity of the treatment regimen gets very high, neither mineral oils nor hydro-gels are sufficient.
In some applications, water is used as a coupling agent. To use water with HIFU, patients are usually partially or wholly submerged in water tanks. Water is well known to have the most desirable properties for use as an acoustic transmission medium. Water is used in large tanks for testing, development and experimentation using ultrasound transducers. In actual therapy procedures, large volumes of water are needed to ensure proper coverage and coupling of the patient and to protect against adverse effects such as skin burns from attenuation of the coupling medium. In these applications, the water used in the water tanks is generally circulated through a pump and connected to a filtration system and a degassing system.
Thus there remains a need for a coupling agent that can be used with HIFU systems using high energy levels beyond those in which hydro-gels or mineral oils are suitable for coupling.
There is also a need for a method of using water as a coupling agent without requiring the bulk of filtration and degassing systems.